This invention relates to optical devices such as semiconductor lasers.
Optical Networks are currently of great interest primarily due to their ability to carry a large amount of information. Particularly significant are Dense Wavelength Division Multiplexing (DWDM) Systems which carry several wavelengths in a single optical fiber. Important components of such systems are Raman fiber amplifiers, which allow long-haul transmission of the signal light without regeneration. These amplifiers operate by pumping of charge carriers in the fiber with pump lasers to cause amplification of the signal light. For systems carrying several channels, more pump power and wider signal gain bandwidth are needed. A key feature of Raman amplification is the ability to increase the gain bandwidth by adding pump power at different wavelengths. Therefore, lasers which can emit light at several selected wavelengths, such as Distributed Feedback Lasers (DFB) are attractive for use in pumping Raman amplifiers.
One of the problems associated with Raman amplifiers is the onset of Stimulated Brillion Scattering (SBS) which generates noise and can reflect a significant fraction of the pump power intended to travel down the fiber. The threshold power at which such scattering occurs is a function of the line width of the pump light. Unfortunately, the line width of a typical DFB laser is small (usually less than 1 MHz), which results in a threshold of only about 1 mW. This severely limits the maximum pump power which can be used for Raman Amplifiers. The line width of the laser may be increased by applying an RF modulation. However, this approach is fairly complex and requires additional space for an RF generator.
It is desirable, therefore, to provide a semiconductor laser with a broadened line width. It is also desirable to have single mode emission from the laser so that the light can be coupled into a single mode fiber.
The invention in accordance with one aspect is an optical device which includes a grating and a waveguide coupled to the grating. The width of the waveguide varies along the length of the waveguide over at least a portion of the waveguide coupled to the grating in such a manner as to broaden the spectral line width of light from the device. In one embodiment, the width is varied according to a sinusoidal function.
In accordance with another aspect, the invention is an optical amplifier and a semiconductor laser coupled to the amplifier. The semiconductor laser includes a grating and a waveguide coupled to the grating. The width of the waveguide varies along the length of the waveguide over at least a portion of the waveguide coupled to the grating in such a manner as to broaden the spectral line width of light from the device. In one embodiment, the width is varied according to a sinusoidal function.
In accordance with another aspect, the invention is a method for forming an optical device which includes the step of forming a grating and a waveguide coupled to the grating and having a length and width over a semiconductor substrate such that the width of the waveguide varies along the length of the waveguide over at least a portion of the waveguide coupled to the grating in such a manner as to broaden the spectral line width of light output from the device region.